One method for cutting spiral bevel and hypoid gears is known as the face-milling method with single indexing. In contrast to the continuous indexing method, a tooth space is always first completely produced using the cutter head in this case, the workpiece is indexed further by one tooth spacing, the next tooth space is then cut in the same way, etc., until the workpiece has become a complete bevel gear. For spiral bevel gears, which were manufactured according to this method, the tooth flanks have a lengthwise curvature in the shape of a circular arc.
In this case, the term “spiral bevel gears” is to include both nonoffset and offset bevel gears. If one wishes to differentiate between the two types, the offset bevel gears are referred to as hypoid gears.
Furthermore, there have historically been two production processes for spiral bevel gears and hypoid gears. In one case, the tooth spaces of the pinion and ring gear of a bevel gear pair are each produced in generating processes, in the other case, the tooth spaces of the ring gear are only produced by plunging the rotating cutter head into the stationary workpiece, while in contrast the pinion gaps are produced in a special generating process using an appropriately tilted cutter head. While in the plunge process, the shape of the blades of the cutter head is transferred to the tooth flanks in that the cutter takes one chip at a time over the entire length and depth of the flank, in the generating process, in which the cutter head and workpiece move in relation to one another in accordance with a specific regularity, the tooth flanks are implemented through enveloping cuts of the individual cutter blades.
The related art also includes other milling processes for spiral bevel gears, one of these is described, for example, in EP 0 883 460 (based on the application WO/US97/02086 and the US priority application 60/015,380), which do not have to be gone into in further detail here, however.
For both important processes, in the Gear Handbook, FIG. 20-2 and FIG. 20-7, machines which operate purely mechanically are described, the generating machine (generator) having a rolling drum or cradle and a separate mechanism for tilting the cutter head. In contrast, modern CNC machines for cutting spiral bevel and hypoid gears, as described, for example, in DE 196 46 189 C2 (corresponds to U.S. Pat. No. 5,961,260) or in DE 37 52 009 T2, may achieve this without a rolling drum and without a tilt mechanism, solely through spatial motions of the tool carrier and workpiece carrier. For the single-indexing method, only five controlled axes are necessary for this purpose, three translational and two rotational. The missing sixth degree of freedom for the general position of a rigid body in space, in this case the cutter head in relation to the workpiece, is the rotation of the cutter head around its rotational axis. It is not necessary as a controlled axis in the single-indexing method, because the cutter head is rotationally symmetric and its drive—independently of the other five axes—is only necessary to achieve a desired cutting speed.
Such CNC machines achieve significantly greater operating speeds than purely mechanical bevel gear cutting machines, while simultaneously having more precise setting and travel motions, and are therefore more cost-effective. In spite of this, it has been shown in the quality evaluation of the cut spiral gears with the aid of pitch measurement that the measured results may frequently not be brought into harmony with the overall precision of the CNC machines. The results are worse than expected, although the pitch measurement device operates perfectly.
Besides the cutting machine, the cutter head is also of decisive importance for the precision of the cut bevel gears. For some time there have already been efforts, therefore, particularly in cutter heads for the single-indexing method, to elevate the positional precision of the cutter used. For example, a device is known from the publication DE 200 19 937 U1 (based on PCT/US87/02083), for optimally aligning stick blades in a cutter head. Nevertheless, it is unavoidable that there is at least one blade on the rotating cutter head which lies radially furthest to the cutter head axis and at least one blade which lies closest to it.
This fact is taken into consideration according to the related art in plunge milling of ring gears in the form cutting method in that at the end of each plunged tooth space, the cutter head performs at least one more complete rotation without infeed. In this way, it is to be ensured that the two blades which remove the most are run through the tooth space at least once and thus all gaps are the same. However, this method may not be transferred to the generating process and is also unsatisfactory in the plunge process because it requires additional processing time and increases the wear of the cutter which does not approach the cut correctly.
Therefore, it is the object of the present invention to implement a method and a machine of the type initially cited in such a way that spiral bevel and hypoid gears may be face-milled more uniformly than before, without an additional time expenditure and this is also reflected in the result of the pitch measurement.